Good post by Andreas Skuban (thank you)
There have been performance-related issues reported with certain VDI environments where the number of virtual machines running on XenServer was limited to about 60 or 65. Reports of poor performance within the user sessions on those 60 or 65 machines was also reported. When more than 65 machines were loaded on a single XenApp host, it has been noticed that the dom0 was at capacity and the PVS driver showed poor throughput of 4MB/s or less. Once 65 VMs were loaded, the user experience dropped dramatically. This scenario is explained in an article on the Citrix Blogs.
Read on here
Storage CH Blog 